Gas component detection apparatus

ABSTRACT

A gas component detection apparatus comprises a first and a second gas sensing elements each composed of a metal oxide which exhibits variable electric resistances according to gaseous components and temperatures of gases to be detected. A catalyst is carried at least by the first sensing element for promoting oxidation reactions of the gaseous components of the gases. A first pair of electrodes are inserted into those portions of the first sensing element which are subjected to catalytic action of the catalyst. Into the portions of the second sensing element which are not subjected to catalytic action are inserted a second pair of electrodes. The first pair of electrodes sense a variation in electric resistances resulting from the gaseous components and temperatures of the gases, while the second pair of electrodes detect an electric resistance variation related mainly upon the gas temperatures. Consequently, an output signal reflecting substantially only the gaseous components of the gases is produced by offsetting both of the electric resistances separately sensed utilizing a suitable electric circuit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of the applicants'earlier U.S. patent application, Ser. No. 751,956, filed on Dec. 17,1976, now Pat. No. 4,099,922 dated July 11, 1978.

BACKGROUND OF THE INVENTION

The present invention relates to a gas component detection apparatus fordetecting the variation in concentrations of gaseous components such asoxygen (O₂), carbon monoxide (CO) and hydrocarbon (HC) of for exampleexhaust gases from an internal combustion engine.

Gas component detection apparatuses have been widely used in manyindustrial fields. Lately, as a countermeasure to cope with the problemof exhaust gases from an internal combustion engine, gas componentdetection apparatus have been employed for determining the air-fuelratio of an air-fuel mixture supplied to the internal combustion engine.

In the case where a catalyst is utilized for purifying exhaust gasesfrom an internal combustion engine, the catalyst cannot exhibit maximumproperties unless the air-fuel ratio of an air-fuel mixture ismaintained constantly at a proper value. However, in an ordinaryinternal combustion engine equipped with a conventional carburetor or afuel injection apparatus, the air-fuel ratio is actually inevitablysubjected to a large variation even when the ratio of an injected fuelto intake air is set to be constant. Consequently, in order to maintainconstantly a proper air-fuel ratio, it is necessary to detect with theuse of gas detection apparatus the air-fuel ratio prior to burning ofthe air-fuel mixture and feed back a signal corresponding to thedetected value to the carburetor or the injection apparatus, therebycontrolling the air-fuel ratio of the air-fuel mixture supplied to theengine.

Gas component detection apparatuses are constructed to determine theair-fuel ratio based on the fact that the variation in concentrations ofgaseous components of the exhaust gases is closely related to variationof the air-fuel ratio of the air-fuel mixture. In this connection,consideration has to be given to the fact that the temperature of theexhaust gases, as well as the concentrations of the gaseous componentsthereof, will vary abruptly and remarkably. It is thus desirable thatthe gas component detection apparatuses be operable with high accuracynotwithstanding such prominent variables.

Heretofore, a gas component detection apparatus has been known whichemploys transition metal oxide. In the case where the air-fuel ratio ofan air-fuel mixture is determined by employing such detection apparatus,a differential operational amplifier which has a non-inverted inputterminal and an inverted input terminal is used. The detection apparatusis mounted for example in an exhaust pipe of an internal combustionengine with the transition metal oxide exposed to the exhaust gases, andthe electric resistance variation thereof is detected. A referencevoltage set by reference resistors is applied to the non-inverted inputterminal of the amplifier, and a voltage established by the electricresistance of the transition metal oxide is applied to the invertedterminal of the amplifier. The amplifier compares the voltages appliedto both its input terminals and produces a corresponding output signal,and the latter signal can be utilized to control the air-fuel ratio ofthe air-fuel mixture supplied to the internal combustion engine.

However, in order to effect proper control of the air-fuel ratio byemploying the abovementioned detection apparatus, it is necessary tocompensate the electric resistance variation of the transition metaloxide due to temperature variation of the exhaust gases since theelectric resistances exhibited by the transition metal oxide varydepending upon not only the concentrations of the gaseous components ofthe exhaust gases, but also the temperature thereof. For example, in thecase where the reference voltage is set to control the air-fuel ratio tothe stoichiometrical one at an exhaust gas temperature of 850° C., thecontrol can be preferably effected at this temperature. However, whenthe exhaust gas is at 350° C., the determined air-fuel ratio is smallerthan the stoichiometrical one, which makes precise control of theair-fuel ratio impossible.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a gascomponent detection apparatus which can detect with high accuracygaseous components of exhaust gases or the like, without beinginfluenced by the temperature of the latter.

It is another object of the invention to provide a gas componentdetection apparatus which can be used for controlling the air-fuel ratioof an air-fuel mixture supplied to combustion devices such as aninternal combustion engine.

It is a further object of the invention to provide a gas componentdetection apparatus which can be easily manufactured.

According to one aspect of the present invention, there is provided agas component detection apparatus comprising a first and a second gassensing elements each including a metal oxide which exhibits variableelectric resistances according to gaseous components and temperatures ofgases to be detected, a catalyst carried by the first sensing elementfor promoting oxidation reactions of the gaseous components of thegases, a first pair of electrodes inserted into the first sensingelement for sensing a variation in electric resistances exhibited atthat portion of the first sensing element subjected to catalytic actionof the catalyst and resulting from the gaseous components andtemperatures of the gases, and a second pair of electrodes inserted intothe second sensing element for sensing an electric resistance variationresulting from mainly the gas temperatures.

According to another aspect of the invention, also the second sensingelement carries a catalyst for promoting oxidation reactions of gaseouscomponents of the gases to be detected. The second pair of electrodesare inserted into those portions of the second sensing element which arenot subjected to catalytic action of the catalyst carried thereon. Thecatalyst carried by the second sensing element serves to maintain thelatter element at substantially same temperature as that of the firstsensing element when both first and second sensing elements are exposedto the gases to be detected. Consequently, the detection apparatus candetect concentrations of the gaseous components with further improvedaccuracy.

The gas component detection apparatus of the invention is preferablyused with an internal combustion engine to determine whether theair-fuel ratio of the air-fuel mixture supplied thereto is larger orsmaller than the stoichiometrical air-fuel ratio, thereby controllingthe air-fuel ratio of the air-fuel mixture supplied into the engine atthe stoichiometrical air-fuel ratio.

The above and other objects as well as novel features and advantages ofthe invention will become more apparent from the following descriptionwhen read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a first and a second gas sensing elements,explaining a principle of a gas component detection apparatus accordingto the present invention;

FIG. 2 graphically shows the relationship between the air-fuel ratio ofan air-fuel mixture and the electric resistances sensed at pairs ofelectrodes incorporated in the first and second sensing elements,respectively;

FIG. 3 is a vertical sectional view of a first embodiment of a gascomponent detection apparatus according to the invention;

FIG. 4 is an enlarged sectional view showing a first and second sensingelements used in the detection apparatus shown in FIG. 3;

FIG. 5 is an explanative sectional view showing the first and secondsensing elements of FIG. 4, with their electrodes not welded together;

FIG. 6 is a schematic diagram of one example of an electric circuit fordetermining and controlling the air-fuel ratio utilizing the detectionapparatus according to the invention;

FIG. 7 is a schematic diagram similar to FIG. 6 showing another exampleof an electric circuit;

FIGS. 8A to 8C are sectional views similar to FIG. 1, showing essentialparts of the gas component detection apparatus according to a secondembodiment of the invention; and

FIG. 9 is a vertical sectional view similar to FIG. 3, showing a thirdembodiment of a gas component detection apparatus according to theinvention.

Same or similar reference numerals are used to designate same or similarparts throughout the figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First of all, the underlying principle of the present invention will bedescribed with reference to FIGS. 1 and 2. Referring first to FIG. 1, afirst gas sensing element 1 is composed of a plate-like sintered mass oftitanium oxide, and carries a catalyst 3 composed of platinum or thelike not only on its surfaces but also within the element 1. Into theportions of the element 1 subjected to catalytic action of the catalyst3 are inserted a pair of electrodes 1a and 1b. A second gas sensingelement 2 is composed of a plate-like sintered mass of titanium oxide asis similar to the first sensing element 1, and a pair of electrodes 1cand 1d are inserted thereinto. The second element 2 carries thereon nocatalyst.

Upon usage, the first and second sensing elements 1 and 2 are exposedfor example to the exhaust gases emitted from an internal combustionengine. As is well-known, the exhuast gases contain gaseous componentssuch as oxygen (O₂), nitrogen oxides (NO_(x)), carbon monxide (CO),hydrocarbon (HC) and hydrogen (H₂), and the concentration of each ofthese gaseous components varies depending upon the air-fuel ratio of anair-fuel mixture prior to burning. In general, sensing elements of thistype exhibit variable electric resistances according to the variation inthe overall exhaust gas condition brought about by variations in thepartial pressures of these individual gaseous components. Further, thistype of the sensing elements are influenced by the temperature of theexhaust gases, and exhibit variable electric resistances according tothis temperature.

When the first sensing element 1 is exposed to the exhaust gases, forexample the following reactions will take place because of the presenceof the catalyst 3.

    CO+1/2O.sub.2 →CO.sub.2

    HC+XO.sub.2 →YCO.sub.2+ 2H.sub.2 O

(In these formulae, X and Y indicate suitable coefficients.) As aresult, the O₂ partial pressure of the exhaust gases changes greatly onthe surfaces of the first sensing element 1 when the surroundingatmosphere changes from the reduction to the oxidation atmosphere andvice versa, and this variation in O₂ partial pressure may be identifiedin terms of the variation in electric resistances sensed between thepair of electrodes 1a and 1b. This variation in electric resistances isremarkable in the vicinity of the stoichiometrical air-fuel ratio.

On the other hand, the variation in the O₂ partial pressure caused onthe surfaces of the second sensing element 2 is not so remarkable asthat of the first sensing element 1 because the second sensing element 2carries no catalyst. Thus, the variation in electric resistances sensedbetween the electrodes 1c and 1d of the second sensing element 2 mainlyreflects the variation in temperature of the exhaust gases.

As will be understood from the foregoing, the electrodes 1a and 1b ofthe first sensing element 1 detect the electric resistance variationwhich depends upon both the gaseous components and the exhaust gastemperature, while the electrodes 1c and 1d of the second sensingelement 2 senses the electric resistance variation resulting mainly fromthe exhaust gas temperature. It should be noted that the value of theelectric resistance variation present at the electrodes 1a and 1bdepending on the exhaust gas temperature variation is substantiallyidentical with the value of the electric resistance variation which issensed between the electrodes 1c and 1d of the second element 2 whichalso has resulted from the exhaust gas temperature variation, since bothelements 1 and 2 are composed of the same metal oxide.

FIG. 2 graphically shows a relationship between the air-fuel ratio ofthe air-fuel mixture and the electric resistances sensed at therespective pairs of electrodes. In FIG. 2, curve I shows therelationship between the air-fuel ratio and electric resistancevariations resulting from both the exhaust gas temperature and thegaseous components, which were sensed between the electrodes 1a and 1bof the first sensing element 1, while curve II shows the relationshipbetween the air-fuel ratio and electric resistance variations dependentmainly upon the exhaust gas temperature, which were sensed between theelectrodes 1c and 1d of the second sensing element 2. These curves I andII were obtained by an experiment in which the sensing elements 1 and 2and the catalyst 3 were composed of titanium oxide (TiO₂) and platinum(Pt), respectively. Additionally, the exhaust gas was at a temperatureof 600° C. In the FIG. 2 graph, the ordinate indicates electricresistance (KΩ) in a logarithm scale, while the abscissa indicates theair-fuel ratio of the air-fuel mixture with an equally divided scale. Asdescribed, the value of the electric resistance variation detected atthe electrodes 1a and 1b and dependent on the exhaust gas temperaturevariation is substantially identical with the value of the electricresistance variation dependent on exhaust gas temperature variationsensed between the electrodes 1c and 1d. Therefore, characteristiccurves similar to the curves I and II of FIG. 2 can be obtained atdifferent exhaust gas temperatures. This means that the variation inelectric resistance sensed between the pair of electrodes 1a and 1b ofthe first sensing element 1 may be compensated by the variation inelectric resistance detected at the pair of electrodes 1c and 1d of thesecond sensing element 2, so as to determine an exact stoichiometricalair-fuel ratio regardless of the variation in temperature of the exhaustgases.

FIGS. 3 to 5 show a first embodiment of the invention. The firstembodiment includes a first and second gas sensing elements 1 and 2 eachcomposed of a plate-like sintered mass of titanium oxide. When preparingthe sensing elements 1 and 2, a powder of titanium oxide (in a rutilestructure) sintered at a temperature of about 1200° C. is finely dividedinto particles having average diameters of 0.1 to 3 microns utilizing aball mill or the like. The finely divided power is then kneaded in akneader with an organic binder solution into a slurry. Thereafter, theslurry is formed by a doctor blade process into sheets each having athickness of about 0.2 mm, and a few of the sheets are overlayed intotwo layers of appropriate thickness with pairs of platinum electrodes1a, 1b and 1c, 1d inserted into the respective sheet layers. The sheetlayers having the electrodes inserted thereinto are then pressed andsintered to form the sensing elements 1 and 2. Catalysts 3 are carriedon and within the first sensing element 1, as further describedhereunder. The electrodes 1b and 1c of the first and second sensingelements and an additional platinum electrode 1e are welded together sothat the sensing elements 1 and 2 may be electrically connected inseries. The catalysts 3 composed of platinum are carried on and withinthe first sensing element 1, for example by impregnating the sensingelement 1 in chloroplatinate (H₂ PtCl₄.6H₂ O), in turn deoxidizing thesame in a hydrogen current and in turn sintering the same.Alternatively, an evaporation process may be used for this purpose. Ineither case, care should be taken so that the electrical short-circuitbetween the electrodes 1a and 1b may be avoided. The outer end faces ofthe electrodes 1a and 1d are exposed to the outer surfaces of thesensing elements 1 and 2, respectively.

The first and second sensing elements 1 and 2 thus prepared are fixed tothe lower end of a refractory, electrically insulative protecting body 4made of for example alumina, and the electrodes 1a, 1e and 1d areinserted into vertical electrode holes 4a formed in the lower portion ofthe protecting body 4. Metal lead wires 5 each having a flange 5a and aknurled portion 5b are inserted into vertical lead wire holes 4b formedin the upper portion of the protecting body 4, and the knurled portions5b of the lead wires 5 are securely jointed to the protecting body 4with a glass ceramic adhesive 5c or the like. The lower ends of the leadwires 5 and the upper ends of the electrodes 1a, 1e and 1d areelectrically contacted with each other in lateral holes 4c in theprotecting body 4 and are securely jointed to each other by the laserspot welding.

The protecting body 4 with the first and second sensing elements 1 and 2incorporated therein is inserted into a housing 6 which is made of arefractory metal and has externally threaded screws 6a for mounting thegas detection apparatus on for example an exhaust pipe (not shown) of anautomotive vehicle. A washer 7 made of a refractory metal and the upperportion of a protecting cover 8 made of a refractory metal are securelyclamped between the lower tapered portion 4d of the protecting body 4and the mating tapered inner surface of the housing 6. The protectingcover 8 is formed with small holes 8a for permitting the flow of exhaustgases into and out of the protecting cover 8. Between the upper end 6bof the housing 6 and the shoulder portion 4e of the protecting body 4are interposed a washer 10 and a ring 9 made of a relatively soft metalsuch as copper. The upper end 6b of the housing 6 is radially inwardlybent toward the protecting body 4 to securely joint together the housing6 and the protecting body 4.

FIG. 6 shows one example of an electric circuit incorporating the gascomponent detection apparatus of the invention. In this circuit, thesecond sensing element 2 not carrying the catalysts is represented by adetector resistor R₃ between the electrodes 1d and 1c, while the firstsensing element 1 carrying the catalysts 3 is represented by anotherdetector resistor R₄ between the electrodes 1a and 1b. The detectorresistors R₃ and R₄ are connected in series, and an intermediate point xis connected to an inverted input terminal (-) of a differentialoperational amplifier C via electrode 1e. Reference resistors R₁ and R₂are connected in series, and a reference voltage established by thesereference resistors is adapted to be applied to the non-inverted otherinput terminal (+) of the amplifier C. At the intermediate point x, theelectric resistance variation sensed across the electrodes 1a and 1b dueto the temperature variation of the exhaust gases is substantiallyoffset or cancelled by the electric resistance variation across theelectrodes 1 d and 1c due to this exhaust gas temperature variation.Consequently, there is established at the intermediate point x a voltagerepresenting only the abrupt electric resistance variation produced bythe gaseous components. In other words, the voltage obtained at theintermediate point x substantially depends upon the concentrations ofthe gaseous components or the air-fuel ratio.

The electric resistance value of the first sensing element 1 willexhibit an abrupt variation when the actual or detected air-fuel ratiois changed from the stoichiometrical air-fuel ratio. Thus, in order tocontrol the actual air-fuel ratio to the stoichiometrical air-fuelratio, the reference voltage established by the reference resistors torepresent the stoichiometrical air-fuel ratio (such reference voltagebeing indicated by a phantom line A in FIG. 2 in terms of electricresistance) is applied to the non-inverted input terminal (+) of theamplifier C. The amplifier C compares the voltage applied to both itsinput terminals, and issues a corresponding output signal for operatingan actuator D comprising for example, an air-fuel ratio compensationunit of a carburetor. In the case where the detected air-fuel ratio islarger than the stoichiometrical air-fuel ratio whereby the voltage atthe intermediate point x is larger than the reference voltagerepresenting the stoichiometrical air-fuel ratio, the amplifier C issuesan output signal for operating the actuator D to make the air-fuel ratiosmaller, thereby reducing it to the stoichiometrical one. On the otherhand, in the case where the detected air-fuel ratio is smaller than thestoichiometrical one and the voltage at the intermediate point x issmaller than the reference voltage, the output signal issued from theamplifier C operates the actuator D to make the air-fuel ratio larger soas to increase it to the stoichiometrical one. In FIG. 6, a character Vdesignates an electric source such as battery.

As will be understood from the foregoing, with the structure of thedetection apparatus according to the invention, the electric resistancevariation exhibited at the first sensing element 1 carrying thecatalysts 3 and that exhibited at the second sensing element 2 notcarrying the catalysts can be detected separately. Thus, it becomespossible to offset the electric resistance variation exhibited at thefirst element 1, due to the exhaust gas temperature variation, by theelectric resistance variation exhibited at the second element 2.Consequently, the precise determination of the actual air-fuel ratio canbe constantly made.

It will be understood that the electric circuit shown in FIG. 6 is onlyone example of a circuit which may be employed. Another electriccircuit, such as that shown in FIG. 7, also may be used. In the circuitof FIG. 7, the reference resistor R₂ and the detector resistor R₄ areconnected in series, while the reference resistor R₁ and the detectorresistor R₃ also are connected in series. The voltage established at theintermediate point x' between the reference resistor R₂ and the detectorresistor R₄ is applied via electrode le to the inverted input terminal(-) of the amplifier C, while the voltage established between thereference resistor R₁ and the detector resistor R₃ is applied to thenon-inverted input terminal (+) of the amplifier C. The remainingstructure of this circuit is substantially similar to the electriccircuit of FIG. 6. Accordingly, the same or similar parts are indicatedby the corresponding reference numerals and characters in these figures.With the circuit of FIG. 7, the electric resistance variation sensedbetween the electrodes 1a and 1c due to the temperature variation of theexhaust gases is substantially offset by the electric resistancevariation between the electrodes 1a and 1b due to this temperaturevariation.

FIGs. 8A to 8C illustrate essential parts of a second embodiment of thegas component detection apparatus in accordance with the invention. Inthis second embodiment, the second sensing element 2 carries catalyst 3aas does the first sensing element. The catalysts 3a are composed ofplatinum or the like. Electrodes 1c and 1d are so positioned that theelectric resistances sensed thereacross may not be affected by thecatalytic action of the catalysts 3a, since the primary function of thesecond sensing element 2 is to detect the variation in electricresistances due to the variation in the exhaust gas temperature. In thefirst embodiment described, a reaction heat resulting from catalyticaction of the catalyst carried on and within the first sensing elementcauses a temperature rise at the surface of the latter. Since the secondsensing element 2 in the second embodiment carries the catalysts 3a, thetemperature rise occurs also at the surface of the second sensingelement 2 due to the catalytic action of the catalysts 3a so that thetemperature difference between the first and second sensing elements 1and 2 may be substantially eliminated. Consequently, the accuracy indetection performance of the detection apparatus may be improved.

FIG. 9 shows a third embodiment of the invention. In this embodiment,one of the electrodes 1b of the first sensing element 1 and one of theelectrodes 1c of the second sensing element 2 are welded to theprotecting cover 8 so that they may be grounded. The third embodimentdoes not comprise an electrode corresponding to the electrode 1e of thefirst embodiment, so that the first and second sensing elements are notelectrically connected. It will be apparant that the detection apparatusof this third embodiment can detect the concentrations of gaseouscomponents by utilizing a suitable electric circuit. The remainingstructure of the third embodiment is substantially similar to the firstembodiment described hereinbefore. It will be understood that the secondsensing element 2 of this third embodiment may carry the catalysts inthe same manner as the second sensing element of the second embodimentshown in FIGS. 8A to 8C.

Although the first and second sensing elements 1 and 2 of theabove-described embodiments are composed of titanium oxides (TiO₂),these sensing elements may be composed of tin oxide (SnO₂).Alternatively, each of these sensing elements may be composed of aplurality of different metal oxides such as zirconium oxide (ZrO₂),nickel oxide (NiO), cerium oxide (CeO₂) and zinc oxide (ZnO). The lattermetal oxides exhibit substantially same resistance variation for thesame temperature variation.

It is preferable that the first sensing element 1 has a porous structureso that the gases to be detected may easily penetrate into this element.With this structure, the first sensing element becomes very sensitive tothe variation in concentrations (partial pressures) of gaseouscomponents of the gases. Also it is preferable that the second sensingelement 2 has a dense structure so as to prevent the gases frompenetrating into this element. This structure makes the second sensingelement substantially free from the influence of the gaseous components.

In order to make the second sensing element substantially free from theinfluence of the gaseous components, it is also effective to addchromium oxide (Cr₂ O₃) or manganese oxide (MnO₂) to the second sensingelement. For example, more than 5 atm % of chromium oxide or more than 1atm % of maganese oxide may be added to the second sensing elementcomposed of titanium oxide. In the case where chromium oxide ormanganese oxide is added to the second sensing element having the densestructure as described above, the properties in this respect of thelatter element is further improved.

The first sensing element may be constituted by a thin film carried on abase body which is composed of a refractory, electrically insulativemetal oxide. In this case, the sensing element is carried thereon as thethin film having a thickness of about 100 angstroms to 100 microns, bymeans of for example vacuum evaporation or spattering process. Thecatalysts are carried on and/or within the thin film for example byelectric beam evaporation. The platinum electrodes may be formed by apaste firing process or an evaporation process. Similarly, the secondsensing element may be constituted by the abovementioned thin filmcarried on the base body.

In order to prevent that impurities such as phosphor, lead and the likein the gases to be detected are attached to the surfaces of the firstand second sensing elements directly exposed to the gases, the lattersurfaces may be covered with electrically insulative and gas permeableporous ceramic films which are composed of for example γ-almina.

The gas component detection apparatus according to the present inventioncomprises two gas sensing elements which function separately incooperation with the corresponding electrodes. This structure makes themanufacture of the sensing elements easier as compared with thestructure in which a single sensing element partially carrying thecatalyst is used. Additionally, the structure of the invention in whichtemperature compensation is effected with the second sensing elementimproves accuracy in temperature compensation, as compared with the casewhere the temperature compensation is effected utilizing a thermocouple,a thermistor or the like. The latter structure is advantageous also fromthe standpoint of prices of the detection apparatus.

What is claimed is:
 1. A gas component detection apparatuscomprising:first and second gas sensing elements each including a metaloxide which exhibits variable electric resistances according to gaseouscomponents and temperatures of gases to be detected; a catalyst carriedby said first sensing element for promoting oxidation reactions of thegaseous components of the gases, a first pair of electrodes insertedinto said sensing element for sensing a variation in electric resistanceexhibited at that portion of said first sensing element subjected tocatalytic action of said catalyst and resulting from the gaseouscomponents and temperatures of the gases, a second pair of electrodesinserted into said second sensing element for sensing an electricresistance variation resulting mainly from the gas temperature, circuitmeans adapted to be joined to said sensing elements for producing anoutput signal, and means connecting the electrodes of the sensingelements to the circuit means in a manner whereby the variations inresistance of the sensing elements resulting from the gas temperaturesoffset one another and said output is representative of variations inresistance of the first sensing element resulting from the gaseouscomponents.
 2. A gas component detection apparatus as set forth in claim1, wherein the metal oxide of each of said first and second sensingelements includes a plurality of different metal oxides, said oxidesexhibiting substantially the same resistance variation for the sametemperature variation.
 3. A gas component detection apparatus as setforth in claim 1 or 2, wherein said first sensing element has a porousstructure, while said second sensing element has a dense structure.
 4. Agas component detection apparatus as set forth in claim 3, wherein saidsecond sensing element includes chromium oxide as an additional oxide.5. A gas component detection apparatus as set forth in claim 3, whereinsaid second sensing element includes manganese oxide as an additionaloxide.